package org.xiaojinlong.algo;

import java.util.Comparator;

/**
 * optimized mergesort
 * @author Jin Long
 *         2015/10/10
 */
public class Merge {
    private static final int CUTOFF = 7;

    private Merge() { }

    private static void merge(Comparable[] src, Comparable[] dst, int lo, int mid, int hi) {
        assert isSorted(src, lo, mid);
        assert isSorted(src, mid+1, hi);

        int i = lo, j = mid+1;
        for (int k = lo; k <= hi; k++) {
            if (i > mid)
                dst[k] = src[j++];
            else if (j > hi)
                dst[k] = src[i++];
            else if (less(src[j], src[i]))
                dst[k] = src[j++];
            else
                dst[k] = src[i++];
        }

        assert isSorted(dst, lo, hi);
    }

    private static void sort(Comparable[] src, Comparable[] dst, int lo, int hi) {
        if (hi <= lo + CUTOFF) {
            insertionSort(dst, lo, hi); // use insertion sort for small subarrays
            return;
        }
        int mid = lo + (hi - lo) / 2;
        sort(dst, src, lo, mid);
        sort(dst, src, mid+1, hi);
        if (!less(src[mid+1], src[mid])) {
            System.arraycopy(src, lo, dst, lo, hi + 1 - lo);
            return;
        }
        merge(src, dst, lo, mid, hi);
    }

    public static void sort(Comparable[] a) {
        Comparable[] aux = a.clone();
        sort(aux, a, 0, a.length-1);
        assert isSorted(a);
    }

    private static void insertionSort(Comparable[] a, int lo, int hi) {
        for (int i = lo; i <= hi; i++)
            for (int j = i; j > lo && less(a[j], a[j-1]); j--)
                exch(a, j, j-1);
    }

    private static void exch(Object[] a, int i, int j) {
        Object swap = a[i];
        a[i] = a[j];
        a[j] = swap;
    }

    private static boolean less(Comparable a, Comparable b) {
        return a.compareTo(b) < 0;
    }

    private static boolean less(Object a, Object b, Comparator comparator) {
        return comparator.compare(a, b) < 0;
    }

    public static void sort(Object[] a, Comparator comparator) {
        Object[] aux = a.clone();
        sort(aux, a, 0, a.length-1, comparator);
        assert isSorted(a, comparator);
    }

    private static void merge(Object[] src, Object[] dst, int lo, int mid, int hi, Comparator comparator) {
        assert isSorted(src, lo, mid, comparator);
        assert isSorted(src, mid+1, hi, comparator);

        int i = lo, j = mid+1;
        for (int k = lo; k <= hi; k++) {
            if (i > mid)
                dst[k] = src[j++];
            else if (j > hi)
                dst[k] = src[i++];
            else if (less(src[j], src[i], comparator))
                dst[k] = src[j++];
            else
                dst[k] = src[i++];

            assert isSorted(dst, lo, hi, comparator);
        }
    }

    private static void sort(Object[] src, Object[] dst, int lo, int hi, Comparator comparator) {
        if (hi <= lo + CUTOFF) {
            insertionSort(dst, lo, hi, comparator);
            return;
        }
        int mid = lo + (hi - lo) / 2;
        sort(dst, src, lo, mid, comparator);
        sort(dst, src, mid+1, hi, comparator);

        if (!less(src[mid+1], src[mid], comparator)) {
            System.arraycopy(src, lo, dst, lo, hi - lo + 1);
            return;
        }

        merge(src, dst, lo, mid, hi, comparator);
    }

    private static void insertionSort(Object[] a, int lo, int hi, Comparator comparator) {
        for (int i = lo; i <= hi; i++)
            for (int j = i; j > lo && less(a[j], a[j-1], comparator); j--)
                exch(a, j, j-1);
    }

    private static boolean isSorted(Comparable[] a) {
        return isSorted(a, 0, a.length-1);
    }

    private static boolean isSorted(Comparable[] a, int lo, int hi) {
        for (int i = lo + 1; i <= hi; i++)
            if (less(a[i], a[i-1]))
                return false;
        return true;
    }

    private static boolean isSorted(Object[] a, Comparator comparator) {
        return isSorted(a, 0, a.length-1, comparator);
    }

    private static boolean isSorted(Object[] a, int lo, int hi, Comparator comparator) {
        for (int i = lo + 1; i <= hi; i++)
            if (less(a[i], a[i-1], comparator))
                return false;
        return true;
    }
}
